1. Field of the Invention
The invention relates to an apparatus for inductively heating a sleeve section, having a central holding opening for a shank of a rotary tool, for example a drill, milling cutter or reaming tool, of a tool holder that holds the tool shank in the holding opening in a press fit and releases it upon heating.
2. Background of the Related Art
Particularly in the case of rapidly rotating tools that run, for example, at speeds of 10 000-20 000 rpm or even higher, it is known to shrink the tool shank into a sleeve section of a tool holder. For this purpose, the sleeve section is heated, usually to a few 100° C., for example 250° C.-350° C., such that the holding opening of the sleeve section widens and the tool can be inserted with its shank into the holding opening. The nominal diameter of the holding opening is somewhat smaller than the outside diameter of the tool shank. After the cooling of the sleeve section, the tool shank is therefore held securely in terms of rotation in a press fit in the tool holder. This shrinking technique permits the tool to be clamped extremely accurately for true running and thus with little unbalance.
A gas flame was firstly proposed as heat source for heating the sleeve section, but thought has also been given to heating collars that are to be brought into bearing contact with the tool holder. Because they permit the heating up phase to be kept very short, attention has more recently been concentrated on inductive heating devices. Such inductive heating devices have so far regularly had an induction coil that is fed from a generator with alternating current or a pulsed direct current and can be mounted centrally on the sleeve section in order to heat it. The magnetic field of the induction coil induces in the electrically conducting material of the tool holder eddy currents that directly heat the sleeve section. In order to be able to use one and the same induction coil in the case of tool holders of different outside diameter of the sleeve section, the induction coil surrounds the sleeve section at an axial spacing in this case.
Usually, the induction coil is surrounded on its outer circumference by a yoke shell of magnetizable material. In addition, there are usually arranged at the axial end faces of the induction coil annular elements that likewise consist of magnetizable material and serve as pole shoes which are situated closely adjacent to the end of the sleeve section on the tool side and the end remote from the tool, in particular to even bear against the sleeve section. Because of their high magnetic conductivity relative to air, the yoke shell and the pole shoes ensure a concentration of the magnetic flux, which is directed, thus focused, onto the sleeve section.